The present invention relates to ultrasound imaging and more particularly to an improved multipulse and enhancement strategy for ultrasound imaging of an object containing an ultrasound contrast enhancement imaging agent.
Ultrasound contrast agents can be introduced into the body to reflect or absorb ultrasound energy, or to resonate when exposed to such energy, and thereby provide an enhanced image of a part of the body. Examples of such contrast agents, in the form of hollow microcapsules, are given in Japanese patent applications nos. 508032/1992 and 509745/1994 and in PCT/GB95/02673 (WO 96/15814). Such agents are injected into the patient""s bloodstream and then the patient is subjected to ultrasound radiation.
In the present invention the ultrasound sequence comprises a multiple sequence comprising a first pulse burst at a first frequency and low amplitude followed by a second pulse burst at second frequency and relatively higher amplitude. This second pulse is of sufficient magnitude to induce power enhanced scattering, as defined, in a region of interest. This is then further followed by a third pulse burst of a third frequency and lower amplitude.
Power enhanced scattering is defined as providing an acoustic pulse at an amplitude at least sufficient to cause a change in the acoustic properties of the region of interest to, for example, cause bubbles to be released from the microcapsules.
The present invention provides a method of producing an ultrasound image of an object containing an ultrasonic contrast imaging agent comprising subjecting the object to a first pulse burst of a first frequency and first power, subjecting the object to a second pulse burst of a second frequency in combination with a second power for optimal bubble release and subjecting the object to a third pulse burst of a third frequency and third power, obtaining a first image of the object as a result of the first pulse burst, obtaining a second image of the object as a result of the third pulse burst and comparing the first and second images to obtain a final enhanced image.
Preferably said first power is a low power relative to said second power which is a high power and said third power is a low power relative to said second power.
Preferably in a first embodiment the first and third pulse bursts are at a frequency higher than that of the second pulse bursts.
Alternatively the first and third pulse burst are at a frequency lower than that of the second pulse burst.
Preferably the first and third pulse bursts are identical or have a defined and known relationship.
Preferably the first and third pulse bursts comprise a relatively lower number of cycles than the second pulse burst.
In a specific embodiment the first and third pulse bursts comprise a single cycle.
Preferably the second pulse burst comprises a plurality of cycles. In a specific embodiment this comprises four or more cycles.
Preferably the time between the first and third pulse bursts is less than 100 xcexcs.
In a specific embodiment the third pulse burst can be combined with or overlap with the second pulse bursts. Any image pulse obtained from the third pulse burst can be filtered out from any interference from the second pulse bursts by virtue of the difference in frequencies.
In the imaging method a first image is obtained during the first pulse burst and a second image is obtained during the third pulse burst. The second higher amplitude pulse burst comprises a release burst for release of bubbles from a suitable agent such as Quantison.
The invention also provides apparatus for carrying out the above method.
Suitable microcapsules include those disclosed as xe2x80x9cQUANTISONxe2x80x9d(trademark) microcapsules by Andaris Limited, and described in WO92/18164 (U.S. Pat. No. 5,518,709), WO94/08627 and WO96/15814 (U.S. Ser. No. 08/676,344 filed Jul. 19, 1996), all of which are incorporated herein by reference. The microcapsules are made by spray-drying a solution of serum albumin to form hollow microcapsules generally of diameter 1 to 10 xcexcm; for example90% may have a diameter of 1.0 to 9.0 xcexcm or 1 to 6.0 xcexcm, as measured in a Coulter Counter Multmizer II. However, any gas containing microcapsule, microsphere or microparticle which releases the gas on irradiation with a non-physiologically harmful dose of ultrasound may be used in the methods of the invention.
In an enhancement sequence the first and second images obtained during the first and third pulse bursts are compared with each other to provide a combined improved image, for example by subtractive decorrelation.